1. Field of the Invention
The present invention relates to a motor.
2. Description of the Related Art
A flexible disk drive, compact disk drive and the like incorporate a magnetic head and a stepping motor. The magnetic head is responsible for writing data on a disk as a recording medium, and reading out data from the disk. The stepping motor has a shaft which has a spiral ridge formed on the surface of its exposed portion and functioning as a lead screw, and when the stepping motor rotates, the spiral ridge as a lead screw works to change the rotational movement of the shaft into the linear movement of the magnetic head. Conventional stepping motors for use in a flexible disk drive, compact disk drive, etc. are disclosed, for example, in Unexamined Japanese Patent Application KOKAI Publication No. H7-75322 (FIGS. 1 and 3), Unexamined Japanese Patent Application KOKAI Publication No. H8-186950 (FIGS. 1 and 7), and Unexamined Japanese Utility Model Patent Application KOKAI Publication No. S63-77471 (FIGS. 1 and 2). And a manufacturing method for a lead screw integrated with a rotor is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H8-118371 (FIGS. 1 to 6).
Referring to FIG. 3 of the present application showing a conventional stepping motor, a rotor comprises a magnet 1 and a shaft 2 passing through the center of the magnet 1. The shaft 2 is fixed to the magnet 1 with a resin 3 which is filled between the magnet 1 and the shaft 2 and then cured. A stator 4 lodges windings 5, has an open space, and encloses in the open space the magnet 1 of the rotor with a constant gap left therebetween. Terminals 6 for supplying current to the windings 5 are attached on the stator 4.
One end of the shaft 2 protruding from the magnet 1 passes through a center hole formed at a circular boss 7. The boss 7 is formed of resin and fixed to the stator 4. The one end of the shaft 2 protruding from the boss 7 has a spiral ridge 8 formed on its outer surface. The spiral ridge 8 is resin-molded simultaneously with the resin 3 which fixedly hold the magnet 1 and the shaft 2 together. The shaft 2 with the spiral ridge 8 functions as a lead screw. The tip of the one end of the shaft 2 having the spiral ridge 8 is in contact with a ball 11 housed in a thrust bearing 10 attached to one (distal) end of a bracket 9 which has the other (proximal) end fixedly attached to the stator 4 by welding or the like.
The tip of the other end of the shaft 2 is in contact with a ball 13 which is in contact with a plurality of balls 14 housed in a cavity formed on a side of a pivot housing 12 which is formed of, for example, resin. On a side of the pivot housing 12 opposite to the side having the cavity, an end cap 15 and a thrust spring 16 are provided. The end cap 5 seals the stator 4, and the thrust spring 16 is held by the end cap 5 and presses the pivot housing 12 toward the thrust bearing 10.
In the conventional stepping motor described above, the shaft 2 is supported only at its both end tips by the thrust bearing 10 and the pivot housing 12 and therefore may contingently bow, which causes difficulties in controlling the position of the magnetic head precisely.
Dimensional accuracy and assembly accuracy are another issue. For example, if the dimensional accuracy of the components for the pivot housing 2 is poor, the rotor may be disposed off center causing noises when rotating. The pivot housing 12, when formed of resin for cost reduction, may suffer deterioration in dimensional accuracy.
And, since the thrust bearing 10 receives the shaft 2 via the ball 11, the length of the shaft 2 must be duly controlled, and the variation in the pressing force of the thrust spring 16 must also be controlled. Specifically, the dimensional accuracy of the shaft 2, the bracket 9, the stator 4 and the pivot housing 12 in the shaft length direction must be controlled and the assembly accuracy of these components must also be controlled. And, the spring constant of the thrust spring 16 must be flattened. Thus, in the conventional stepping motor shown in FIG. 3, the components must be produced with high accuracy therefore hindering cost reduction, and the magnetic head position cannot be reliably controlled unless satisfactory assembly accuracy is ensured.
FIG. 2 of the aforementioned Unexamined Japanese Patent Application KOKAI Publication No. H7-75322 and FIG. 7 of the aforementioned Unexamined Japanese Patent Application KOKAI Publication No. H8-186950 show the structure of another conventional stepping motor, in which a sleeve bearing is provided at a portion where a bracket is attached to a stator. And, FIG. 2 of the aforementioned Unexamined Japanese Utility Model Patent Application KOKAI Publication No. S63-77471 shows the structure of still another conventional stepping motor, in which a ball bearing is provided a portion where a bracket is attached to a stator. With a sleeve bearing or ball bearing thus provided, the problem of the shaft bowing is eliminated, and the dimensional accuracy does not have to be controlled so strictly as done in the stepping motor described above with reference to FIG. 3 of the present application. However, the sleeve bearing or ball bearing must be prepared additionally and attached separately, therefore prohibiting cost reduction.
The problems mentioned above are present not only in a stepping motor to rotate a lead screw thereby controlling the position of a magnetic head but also in other motors, thus failing to provide technically satisfactory motors.